<p>Modeling and optimization of complex systems traditionally have required significant programming efforts in order to encode the model dynamics, the cost functions and the constraints in a format suitable for state of the art numerical algorithms. The availability of dedicated languages for physical modeling has made the design process simpler, but often tools have been limited to a particular optimization algorithm. In this paper, we present a case study where a combined cycle power plant model has been developed using first principles in the modeling language Modelica. Based on the model, an optimal control problem for the start-up of the power plant has been formulated in the Optimica extension and solved using the open source software JModelica.org. The results demonstrate how high-level modeling languages and tools can be used to bridge the gap between the engineering need for intuitive description formats and the interfaces of efficient numerical algorithms.</p>